An insulated electrical wire may be used in equipment such as heating equipment or a fire alarm, which requires safety at a high operating temperature. Such an insulated wire is also employed in an automobile in an environment which is heated to a high temperature. An insulated wire of this type is generally formed by a conductor which is coated with heat resistant organic resin such as polyimide or fluororesin.
When an insulated wire is used where a high heat resistance or a high degree of vacuum is required, it is impossible to attain a sufficient heat resistance or non-outgassing property with only the aforementioned organic coating. In that case, therefore, an insulated wire is used that has a conductor which passes through an insulator tube of ceramics, or an MI (mineral insulated) cable comprising a conductor which passes through a tube of a heat resistant alloy, such as stainless steel alloy, filled with fine particles of a metal oxide such as magnesium oxide, or the like.
On the other hand, a glass braided tube insulated wire employing an insulating member of glass fiber fabric or the like is known as an insulated wire having a high heat resistance and flexibility.
However, an insulated wire coated with organic resin can maintain its insulability merely up to a temperature of about 200.degree. C. at the most. Therefore, such an insulated wire cannot be used when an insulability is required under a high operating temperature of at least 200.degree. C.
Further, the insulated wire which has an improved heat resistance due to an insulator tube of ceramics has an inferior flexibility. On the other hand, the MI cable, which is formed by a heat-resistant alloy tube and a conductor, has an increased outer diameter. Thus, the MI cable has a relatively large cross-section with respect to electric energy which is allowed by the conductor to pass through the heat-resistant alloy tube. While it is necessary to bend the heat-resistant alloy tube to a prescribed curvature in order to wind the MI cable into a coil or on a bobbin or the like, such bending required for the winding is difficult. When the MI cable is coiled, further, it is difficult to improve the winding density due to the large diameter.
When the glass braided tube insulated wire is arranged in a prescribed configuration, the glass fiber generates glass dust, which may serve as a gas adsorption source. When the glass braided tube insulated wire is employed in environment which requires a high degree of vacuum, it is impossible to maintain the high degree of vacuum due to the gas adsorption source provided by the glass dust.